Electromagnetic energy can propagate through either a transmission line or through free space. Electromagnetic energy which propagates through a transmission line can be transmitted into free space by an antenna that is attached to the transmission line. Electromagnetic fields travel from a transmission line into an antenna through the feed point of the antenna. The power traveling down the transmission line induces a voltage across the feed point of the antenna. This voltage creates currents flowing on the antenna that, in turn, allow the electromagnetic energy to radiate into free space. Once in free space, the electromagnetic energy can propagate without the benefit of transmission lines. Antennas are designed to efficiently couple electromagnetic energy into free space with specified characteristics.
Any transmission line will have a specified characteristic impedance. The maximum amount of electromagnetic energy is transferred from the transmission line to the antenna when the input impedance of the antenna matches the input impedance of the transmission line. Usually, a transmission line has an impedance of 50 Ohms. Thus, to maximize electromagnetic energy transfer, an antenna connected to such a transmission line would have an input impedance of 50 Ohms. Likewise, a transmission line with a 300 Ohm impedance would require an antenna with a 300 Ohm impedance to maximize electromagnetic energy transfer.
The material from which the antenna is constructed will determine how much electromagnetic energy will radiate from the antenna. If the antenna is constructed from lossy (i.e. power absorbing) materials some of the power will be absorbed and the rest will radiate into free space. Measurements of the amount of radiation from an antenna made from a lossy material are used to create a gain pattern for the antenna. The gain pattern is a graph of the magnitude of the electromagnetic energy radiated from the antenna, typically measured in decibels, as a function of the angle around the antenna, typically measured in polar coordinates.
Most commonly, an antenna is assumed to be lossless, and the geometry of an antenna will determine the direction that electromagnetic energy will radiate from the antenna. In a lossless antenna, all of the electromagnetic energy that is input into the antenna is radiated into free space by the antenna. Thus, a lossless antenna neither creates nor destroys electromagnetic energy, but rather merely directs the electromagnetic energy. Thus, in the case of an antenna made from lossless materials, the gain pattern graphs the effect of the geometry of the antenna on the amount of electromagnetic energy radiated in any given point around the antenna. This gain pattern constructed for an antenna constructed from lossless materials is called the directivity pattern of the antenna.
The first antennas were dipole antennas and loop antennas. Dipole antennas are omni-directional and in their most basic form are constructed with two wires that are laid end-to-end. Each of the two wires is connected at one end to one of the conductors of a transmission line. When the transmission line is energized, a voltage appears across the feed point of the antenna. The voltage induces current on the two poles of the antenna referred to as a dipole.
As proven by image theory, one element of the dipole antenna can be replaced by a large metallic sheet to produce a monopole antenna having the same characteristics as one element of the dipole antenna. The well known concept of image theory is applied by antenna designers to create a monopole antenna from a dipole antenna. An antenna designer can create a monopole antenna having an electromagnetic field radiation pattern which is identical to the electromagnetic field radiation pattern of one pole of a dipole antenna by replacing one side of a dipole with large conductive plate that approximates a perfect electrical conductor plane. A perfect electrical conductor plane has no loss, and most metals are a good approximation to a perfect electrical conductor plane so long as the sheet of metal is large in comparison to the electromagnetic field radiation wavelength of the antenna. The currents are imaged through a perfect electrical conductor plane like a reflection in a mirror. The electromagnetic fields produced by currents generated by the monopole are equivalent to the electromagnetic fields produced by the original currents generated by the dipole antenna above the plane of the perfect electrical conductor plane.
Over the years, the shape of the dipoles used in a dipole antenna has evolved to increase the relatively narrow operational frequency band of the wire dipole antenna. Dipoles have been shaped like a bow-tie (two triangles), a bi-cone (two cones), and a fat dipole (wide pieces of copper tape). Broadband antennas are designed to work over a large frequency band. Typically these include helical antennas, bi-conical antennas, spiral antennas, log-periodic antennas, and tapered slot antennas. Tapered slot antennas have fat dipoles which smoothly taper down to a slot positioned between the two dipoles. The slot itself can take a variety of shapes including linear, exponential, or circular shapes. The smooth taper of the fat dipole toward the slot allows the antenna to operate over a large bandwidth.